In general, modern homes are provided with telephone service by a telephone company which provides a subscriber line drop to the subscriber's house. In such a conventional system, one or more individual telephone sets may be connected to the subscriber line in parallel, as individual extension sets, so that each of the individual sets may communicate on the line. The telephone sets are generally connected together with a common cable extending from the subscriber line and having at least two conductors.
A problem with the common residential telephone system is that intercom telephone communication between the various telephone sets on a subscriber line drop is generally not possible. Although conference calls are possible by having each user at a different one of the extension telephone sets, it is generally not possible to use the ringing features of telephone sets to set up such conference calls.
Of late, it has become more common for houses to be equipped with more than one subscriber telephone line. The simplest way to arrange such systems in which multiple subscriber lines are provided is to provide entirely separate wiring and telephone sets for each of the subscriber lines; thus, effectively isolating each subscriber line and its associated telephone sets from each of the other lines and their sets. A problem with such a system which is separated into independent parts is that wasteful duplication of telephone sets may result, and the conferencing of telephone calls between more than one subscriber line is generally not possible. Another problem with such a separated arrangement is that separate sections of interconnection wiring must be added to the house each time that an additional subscriber line is installed.